1. Field of the Invention
The invention relates in general terms to a process for producing organic-light-emitting diodes and to an organic light-emitting diode (OLED). In particular, the invention relates to organic light-emitting diodes with a structured luminous area, and to a process for producing diodes of this type.
2. Description of Related Art
OLEDs are generally built up from an assembly of layers, i.e. a layer structure having an organic electroluminescent layer between two electrode layers, which is applied to a suitable substrate. In general, in an OLED, in each case one of the conductive layers acts as a cathode and the other as an anode. For this purpose, it is known to make the electrode layers from materials with different work functions, so that a work function difference is formed between these layers.
Organic light-emitting diodes (OLEDs) are distinguished by particular advantages over other luminous means. For example, OLEDs have highly promising properties for flat screens, since they allow a significantly wider viewing angle than, for example, LCDs or liquid crystal displays, and as self-illuminating displays also allow reduced current consumption to be achieved compared to backlit LCDs. Moreover, OLEDs can be produced as thin, flexible films which are particularly suitable for specific applications in illumination and display technology.
However, OLEDs are not only suitable for displays. They can be used in general terms as luminous means for a very wide range of applications, such as for example for self-illuminating signs and information boards.
However, the majority of these applications require structured luminous areas. Accordingly, local, fixed-position brightness differences have to be produced on the luminous area. To do this, there are in principle the following options:                The light emitted is modulated directly by laterally structured masking or filtering. For this purpose, an OLED may, for example, have diaphragms, perforated masks, opaque or colored coatings or films on the outer side of the OLED, absorptive or differently colored regions of the substrate on the outer or inner side.        The light emitted is modulated indirectly by the local current density being influenced by the organic electroluminescent layer. This can be achieved, for example, by suitable lateral structuring of the electrodes. It is also possible for the flow of current through the layer system of the OLED layer assembly to be interrupted by the additional presence of insulator structures or structures with a higher resistance in the layer assembly.        Moreover, it is also possible for the electroluminescent layer itself to be laterally structured.        
Local direct modulation of the light flux is in widespread use for the production of LCD or OLED displays in which a broadband light spectrum, preferably white light, is tailored to the desired color loci by means of additional color filters in the light path. In addition, liquid crystal technology requires polarized light which is produced by suitable filtering of the incident light by means of polarizing sheets. A drawback of this is that the light has to be generated and is then partially absorbed again before it emerges from the component. For example, in LCD backlight displays, 60% of the light which is generated within the display is absorbed again in the polarizing sheets alone.
On the other hand, the structuring of the electroluminescent is layer requires special production techniques. For example, organic electroluminescent materials with molecular weights of <1000 amu, known as small molecules, which are suitable for evaporation coating are deposited in structured form by evaporation coating via shadow masks using the PVD process. However, in this case a large proportion of the very expensive electroluminescent material is deposited not on the substrate but rather on the shadow masks. Moreover, when carrying out structuring of this nature, the problem generally arises that, on account of shadowing effects and therefore the partial absence of a dielectric electroluminescent layer, short circuits can occur between the electrode layers, requiring considerably more complex layer structures and therefore more expensive production processes to prevent these short circuits.
By contrast, it is simpler to structure the electrodes or the electrode layers, for example by structured deposition of the electrode materials by means of shadow mask techniques using the PVD process. However, in this case too there is a risk of short circuits at the sharp edges of the electrode structures. A further drawback consists in the fact that in this technology all the structures of the electrode layers must be electrically conductively connected to one another. This problem can only be resolved to an inadequate extent by drawing conductive bridges between isolated structures of an electrode layer. These bridges have an adverse effect on the appearance, may even be illuminated themselves, and the current density passing through a bridge is significantly higher than the current density passing through large-area structures, resulting in a considerable voltage drop across a bridge, which in turn can lead to a lack of homogeneity in the illumination.
By contrast, the most simple option is to apply additional insulating structures in the OLED layer assembly. Possible production processes include evaporation coating through shadow masks or structured adhesive films. However, evaporation coating, sputtering or other PVD processes are complex and expensive vacuum-based processes. On the other hand, adhesive films typically have thicknesses in the region of 10 μm and are therefore generally significantly thicker than the layers of the OLED layer assembly, which are often in the region of only 0.1 μm. The films therefore considerably disrupt the microstructure of the OLED layer structure.
WO 9803043 proposes a process in which a structured insulator layer is applied by photolithography in order to produce a structured luminous area. For this purpose, by way of example, a photomask is produced by means of a printer. A photoresist is applied to the substrate, which has been coated with an indium tin oxide layer, and is then exposed and developed through the mask. In this process, therefore, the pattern which is to be reproduced on the luminous area cannot be transferred directly to the substrate. Rather, the pattern which is to be reproduced has to be produced in a number of intermediate steps on the substrate. When patterns are being produced in photoresist by means of photolithography, a further drawback arises. The edges of the resist structures are once again very sharp. This increases the risk of short circuits. A further particular drawback is in particular the fact that, on account of the sharp-edged structures on a substrate, cords and bubbles may form in layers which are subsequently applied by means of spin coating or dip coating, since the liquid film tends to become detached at these edges, so that its thickness lacks homogeneity. In this context, even subsequent rounding of the edges, as proposed in WO 9803043, provides only a little assistance, since the inner edges which adjoin the substrate remain in place.
A similar process is also proposed in JP 07-289988, in which a homogenous film is applied and is then structured by being exposed and developed, the result obtained being a structured polyurethane film on the electrode layer. Accordingly, the process described in this document has similar drawbacks to the production process disclosed by WO 9803043.
Furthermore, U.S. Pat. Nos. 5,660,573 and 3,201,633 describe electroluminescent capacitors in which the local brightness is likewise influenced by a structured dielectric interlayer. In this case, however, the brightness is influenced not by interrupting a flowing current but rather by influencing the local field strength by means of a lateral variation in the dielectric constant.
However, an electroluminescent capacitor has serious drawbacks compared to an organic light-emitting diode. The light yield is significantly worse. Moreover, operation of an electroluminescent capacitor requires high-frequency alternating current in order to sufficiently strongly excite the electroluminescent material. Provision of this is significantly worse compared to the low DC voltage which is suitable for operation of an OLED. Moreover, the high-frequency alternating voltage at the large-area electrodes leads to strong electromagnetic fields being radiated and/or to a considerable reduction in efficiency of the components as a result of reactive power losses.